How SharePoint 2013 Provider Hosted App get authenticated ?

Apr 10 2015 2:14 PM

1. Can someone explain the full story of what happens when the App Redirect Url is clicked ?

I want to know how the Access Token gets generated using the 2013 TokenHelper class and SharePointContext class and what is the role of Security Token Service ? How the handshake / trust happens between SharePoint and App and can we validate the access token before doing an execute query ?